The Jewish Calendar and Holidays (incl. Sabbath): The Jewish Calendar: The Accuracy of the Jewish Calendar:
How accurate is the Jewish calendar?
The Jewish calendar is based on a figure for the length of the solar year that is 6 or 7 minutes too long. As a result, over the past 1500 years (since the time of Hillel II) the calendar has drifted by about 7 days, and in some leap years the first day of Passover occurs as late as April 25, which is more than a month after the vernal equinox. This discrepancy is not mentioned in most places that discuss the Jewish calendar, but it is treated on pp.207-8 of W.M.Feldman's book Rabbinical Mathematics and Astronomy.
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